Intraocular lenses (IOLs) comprising an optic and, perhaps, one or more haptics for positioning the optic within an eye are known. One type of IOL providing a range of vision including distance vision, intermediate vision and/or near vision is multifocal lenses. Conventional multifocal lenses typically fit into one of two classes.
The first class of multifocal lenses is referred to as refractive multifocal, in which an optic is divided into multiple refractive zones and light from a particular zone is directed to only one of the foci using only refractive power. The zones can be concentric about the optical center or non-axis-symmetric. Refractive multifocal lenses form two or more foci to provide far, intermediate and/or near vision.
The second class of multifocal lenses is referred to as diffractive multifocal. Such lenses include a diffractive element comprising radial zones that transmit light that is out of phase with light transmitted through adjacent zones (i.e., there is a phase delay between adjacent zones). Like refractive multifocal lenses, diffractive multifocal lenses form two or more foci to provide far, intermediate and/or near vision. In diffractive multifocal lenses, the radial boundaries that separate the zones are chosen to achieve particular optical powers.
Both diffractive multifocal and refractive multifocal lens techniques for extending range of vision have resulted in IOLs having distinct foci where vision is sharp, and regions of poorer focus between the foci. A well-known example of a figure of merit for measuring the performance of visual systems is known as a Modulation Transfer Function (commonly referred to as an “MTF”). An MTF of an optical system is a measure of the proportion of contrast of an input object that the optical system is able to maintain when an image of the object is produced. An MTF can be measured as a function of spatial frequency (e.g., line pairs per mm at the retina). Generally, the MTF values for a given optical system decrease with an increase in the spatial frequency.
For a given spatial frequency, each foci of a IOL (i.e., near, intermediate or far focus) manifests itself in a through-focus MTF plot as a peak in MTF values, with regions of lower MTF values between the peaks. For an individual wearer of an IOL, a region of lower MTF values may be large enough to permit vision depending on the broadening and flattening of MTF peaks that occurs for the individual due the ocular aberrations of the individual's eye and his/her pupillary response.
While multifocal lenses are known to provide a beneficial increase in the range of vision of a wearer, a significant proportion of wearers of IOLs employing these multifocal techniques have been known to suffer visual confusion and photic phenomena (i.e., unwanted artifacts in an image formed by pseudophakic eyes) due to the presence of multiple sharply-focused images simultaneously formed on their retinas.
As an alternative to multifocal lenses, techniques for extending the depths of focus of monofocal IOLs (i.e., without multiple peaks in the MTF curve) to obtain distance vision as well as nearer vision have been proposed. IOL techniques to provide an extended depth of focus (EDOF) include: a) providing an IOL with a central refractive add zone; b) providing an IOL with high magnitude positive or negative spherical aberration; and c) providing an underlying refractive IOL with a relatively low-power add diffractive profile (i.e., a diffractive add of 1.5 Diopter or less). Each such extended depth of focus technique has provided limited improvement to wearers' visual quality.
Conventionally, low-power, diffractive add profiles have been selected such that the phase delay between adjacent zones is 0.5 wavelengths of a design wavelength (e.g., approximately 550 nm for visible light). One example of such a lens is described in U.S. Pat. No. 8,747,466. Such lenses tend to provide a high degree of multifocality, such that light is evenly divided between a central focus corresponding to a zeroth order of the diffractive profile, and a near and a far focus corresponding to a +1 order and a −1 order of the diffractive profile, respectively. Such lens configurations tend to cause multiple peaks in the MTF. However, even when the IOL is designed such that the MTF is flattened to eliminate peaks, such designs tend to direct light symmetrically about a central focus to each of the near and far foci, resulting in inefficient use of light energy, without a peak at far vision, and lens performance may be compromised.
According to other diffractive design techniques, the phase delay between adjacent zones has been decreased to a value between 0.4 and 0.5 of a wavelength. Such designs tend to reduce the bifocality of the lens by decreasing the percentage of light sent to the near focus in favor light sent to the far focus since wearers of multifocal lenses tend to prefer peak vision performance for distance vision. Such lenses suffer from similar drawbacks as the more bifocal lenses with regard to presence of multiple peaks in MTF.
Accordingly there remains a need for alternative techniques for extending the depth of focus of ophthalmic lenses without multiple peaks in the MTF curves of resultant lenses and more efficient use of light energy.